In 1979/80, both BA and AF extended their 3 x weekly IAD route to DFW, temp US registrations had to be displayed for this, being overland max speed was Mach 0.95, a time saving but not an efficient way to use the aircraft.
Braniff flight crews operated the IAD-DFW leg.

In 1988, BA again extended the IAD route to DFW, this time US reg's were not needed.

Neither were they needed when between 1984-1991, BA extended the IAD service to MIA, this was a supersonic route however.

Some info in the BA/SQ flights, between BAH-SINSQ flight numbers were used, BA flight crews, 50% of the cabin were SQ;